Induction heating with an internally cooled conductor having a triangular cross-section



Oct. 23, 1962 J. M. KENNEDY, JR 3,069,067

INDUCTION HEATING WITH AN INTERNALLY COOLED CONDUCTOR HAVING A TRIANGULAR CROSS-SECTION Filed Nov. 16, 1959 2 Sheets-Sheet l E2 I u.

INVENTOR dug es M. Kennedy,Jr.

ATTORNEYS 1952 J. M. KENNEDY, JR

INDUCTION HEATING WITH AN INTERNALLY COOLED CONDUCTOR HAVING A TRIANGULAR CROSS-SECTION 2 Sheets-Sheet 2 Fild Nov. 16, 1959 INVENTOR James M. Kennedy, Jr.

ATTORNEYS United States Patent of Maryland Filed Nov. 16, 1959, Ser. No. 853,259

4 Claims. (Cl. 148-13) This invention relates to induction heating and, more particularly, to the induction heating of metal tubes.

The apparatus generally used for induction annealing of metal tubes comprises an induction coil within which the tube is located. The tube is either held stationary for a prescribed period of time or is advanced through the coil at a relatively slow rate of speed while it is being annealed. In the heating of any metal by the induction method, the heat is generated by the alternating current induced in the metal. It is also general practice in induction heating to utilize alternating currents of varying degrees of frequency from 60 cycles through the available radio frequency range. The choice of proper frequency depends on a great number of factors determined by the particular heating application. However, as the frequency of the induced alternating current is'increased, the amount of penetration of this current into the metal becomes less. The heat generated at this outer skin must therefore flow through to the inside of the material by thermal conduction.

In order to properly anneal a large volume of metal at a high temperature, as in the case of metal tubes, it is general practice to establish the induced heating current by means of a relatively long, multiple-turn annealing coil. However, in operation of any such long, multipleturn induction coil, the area of the metal heated to a high temperature by the induced current is considerable. Inasmuch as thermal radiation occurs at the outside surface where the heating effect of the induced current takes place, the rate of heat loss through thermal radiation from this relatively large surface area of the tube represents a substantial proportion of the power input to the tube being heated.

I have now devised a novel method of induction heating in which a single-turn, high-output coil is used to induce an intense, concentrated current in a continuous transverse line around a tube While this tube is passing continuously through the coil. Inasmuch as the current path is narrow and the area at an elevated temperature is minified, radiant heat losses are considerably reduced. The overall efficiency of the induction heating is thereby increased and a greater amount of metal can be heat-treated in a specific time interval without a corresponding increase in the amount of electrical power required. a

My invention thus constitutes an improvement in the induction heating of a metal tube in which the tube is moved longitudinally through a surrounding induction coil in which a high frequency current flows to induce a corresponding heating current in the tube. The improvement pursuant to my invention comprises inducing in the tube a concentrated current in the form of an annular transverse path around the surface portion of the tube so that the width of the effective heating current path longitudinally of the tube is substantially less than the corresponding width of the current-inducing coil.

The apparatus of my invention includes conventional guide means for guiding longitudinal movement of the tube, an induction coil surrounding the coil and a source of high frequency current connected to the coil. .My improvement in this apparatus comprises an induction coil consisting essentially of a single turn of a hollow con- Patented Oct. 23, 1962 ductor having a triangular cross-section positioned with the apex of the triangle facing the tube.

These and other novel features of the method and apparatus of my invention will be more readily understood from the following description taken in conjunction with the drawings in which:

FIG. 1 is a side elevation of the triangular cross-section coil of the invention;

FIG. 2 is a front elevation, partly in section, of the coil of FIG. 1; and

FIG. 3 is a side elevation in section showing induction heating apparatus embodying the induction coil of my invention.

As can be seen in FIG. 1, the inductor coil 5 is in the form of a single turn having a triangular cross-section. The coil is hollow so as to provide a water-cooling passage 6 within its interior. If necessary, a suitable low temperature refrigerating medium can be used instead of water in order to more effectively remove the heat generated in the inductor coil; the use of such a low temperature refrigerating medium insures that the resistivity of the copper inductor is maintained at a low level. The coil 5 is supported by two legs 7 each of which is also provided With an internal water-cooling passage 8 communicating with both the passage 6 of the coil and with water inlet openings 9 in each of the base plates 16 on which the supporting legs 7 are mounted. The base plates are provided with bolt holes 11 so that they may be secured to the induction current generator and thus supply a high frequency current which flows from one base plate 10 through the connecting supporting leg 7 to one end of the coil 5, thence from the other end of the coil through the other supporting leg 7 to the other base plate 10.

It will be noted that the triangular cross-section of the inductor coil 5 is such that the apex of the triangle faces the tube to be heated by passing through the center of the coil. The diameter of the apex portion of the coil is also selected so that it will be as close as possible to the outer wall of the tube to be heated. A close tolerance can be insured and a short circuit from the tube to the inductor can be avoided by interposing an insulating sleeve, of a ceramic for example, between the outer surface of the tube and the apex portion of the coil.

As a result of the triangular shape of the inductor coil 5 with the apex facing the tube to be heated, the current induced in the coil is concentrated in such a narrow annular band about the surface of the tube that, relative to the width of the single coil 5, it can aptly be described as a transverse line about the tube. The concentration of the heating current in such a narrow band or line minifies the area of the elfective heating current at the surface of the tube and thus reduces the amount of heat loss by radiation which normally accompanies an induction heating operation. The frequency of the current supplied to the inductor should be within the range of 10 to 500' kilocycles per second, depending upon the electrical conductivity of the material being heated, in order to cause, by the proximity effect of parallel conductors, an indluced current path close to the inducing current in the cm I have also found it to be advantageous, in order to further increase the efficiency of the apparatus, to surround the inductor coil 5 by a suitable magnetic core of high permeability. This core material should have inherently low hysteresis and eddy current losses at high frequencies, such for example as a sintered compound of magnetic oxides of which the ferrites are particularly effective. The magnetic core acts to increase the completeness with which the magnetic field established by the current in the inductor coil 5 follows the same path as the magnetic field established by the induced current in the tube. As these paths of the magnetic field (the flux) become more completely linked, the inductive reactance set up by the current in the tube more effectively cancels the inductive reactance caused by the current flow in the inductor. The overall inductive reactance of the inductor and tube combination is, therefore, reduced and a greater flow of current in the inductor is possible without increasing the voltage applied.

The use of such a magnetic core with the inductor coil of my invention is shown, by way of illustration, in FIG. 3 which depicts a representative induction heating installation embodying the inductor coil for annealing a metal tube 12. The apparatus of this installation includes a pair of driven entry rolls 13 which serve to drive the tube along its axis past the succeeding components. it should be noted that in the case of these rolls and each of the succeeding roll elements, only one such roll appears in the drawing which is a median vertical section of the side elevation in which only the tube is shown in full. Following the entry rolls the tube passes through idling guide rolls 14 which maintains alignment of the tube 13 as it passes through the high-conductivity copper inductor coil 5. The triangular cross-sectioned coil is positioned with its apex so that the induced current will be established in a narrow band or line running transversely of the tube and of a width substantially less than the width of the coil at the base of the triangle. The high frequency current is supplied to the coil through a conventional high frequency transformer which is connected by water-cooled bus leads to a conventional high frequency power source. In the case of lower frequencies the induction coil can be connected directly to the power generator, inasmuch as the generator has the necessary low-voltage high-current characteristics which make the use of a transformer unnecessary. Separation of the tube from the closely positioned apex of the coil 5 is insured by a ceramic guide sleeve 15. The coil 5 is surrounded by a magnetic core 16 composed of sintered magnetic oxides encased in an insulating jacket 17 of cased epoxy compound or other suitable material. The core is advantageously provided with tubular portions 18 through which a cooling fluid such as water is forced as in the case of the coil itself. Beyond the coil 5 the tube passes through ceramic-faced second idler guide rolls 19.

The next element of the inducting heating installation is purely optional but is particularly advantageous for use in conjunction with the inductor coil of the invention. This element is a water spray 2 0 enclosed in a chamber 21 provided at its leading face with a dished water spray baffle 22. The exit end of the spray chamber communicates with an air wiping chamber 23 having air jets 24 and a wiping washer 25, and the tube leaves the apparatus through driven exit rolls 26. All of the foregoing elements are mounted on a bed plate 27. The water spray serves to quench the heated tube leaving the annealing Zone within the coil. By supplying sufficient water in the spray, and by placing the spray close to the exit end of the coil (with only the ceramic-faced guide roll 1% therebetween), I have found it possible to limit the heating of the tube to only a shallow depth of the wall when the material being heated has a relatively low thermal conductivity, or has a thick wall, or both, which prevent the temperature from becoming rapidly equivalent throughout the wall before the material reaches the water spray. Thus, the restricted heating area of the transverse heating current line established at the surface of the tube pursuant to the invention, together with the rapid quenching action of the water spray, produces a degree of control over the annealing depth not heretofore achieved.

The following example is illustrative of the practice of the invention,

4 Example I Material annealed: 1' OD. X .040" thick wall, deoxidized copper tube.

Travel of tube: 7.6 ft. per min.

Power source: 450 kilocycle oscillator with 50 kw. nominal maximum output capacity (operating at about A. capacity).

Current level in inductor: Approx. 1750 amperes.

Tube heated to: Approx. 1000 F.

Thermal efliciency: Approx. 30%. (Rate at which heat is produced in tube+heating rate equivalent of electrical power output of oscillator.)

Condition of metal before annealing: Wrought, cold worked structure, hardness on Rockwell F scale98.

Condition of metal after annealing: Recrystallized, annealed structure, hardness on Rockwell F scale-50.

Clearance between inductor and tube: A

The practice of the invention makes it possible to obtain an extremely small grain size in the annealed material. This fine grain results from the unusual rapidity with which the material is heated to the recrystallization temperature and can subsequently be cooled, as will be seen from the following example:

Example II The material: Annealed 1%" OD. x .040" wall cartridge brass tube.

Condition of metal before annealing: Wrought cold worked structure.

Hardness on Rockwell F scale: F109 to F110.

Average grain size: .005 mm.

Condition of metal after annealing: Recrystallized annealed structure. Hardness on Rockwell F scale: F66 to F68. Average grain size: .008 mm.

: annealed in a conventional furnace to a Rockwell hard ness of F66 to 68 is .045 mm.

It will be appreciated, accordingly, that the practice of the invention makes possible relatively high efficiency in the induction heating of metal tubes, rods and the like.

1 This high efficiency is attributable to the fact that a very narrow band or line of heating current is established in the surface of the metal so that the rate of heat loss by radiation is lessened with respect to conventional induction heating operations. The method and apparatus of the invention are also characterized by flexibility of operating conditions so that virtually any desired heat treatment can be achieved by control of the power input to the coil and of the rate of travel of the metal through the inductor coil, and the practice of the invention further makes it possible to obtain an extremely small grain size in the annealed metal.

I claim:

1. In the induction annealing of a metal tube in which the tube is moved longitudinally through a surrounding induction coil in which a high frequency current flows to induce a corresponding heating current in the tube, the improvement which comprises inducing in the tube a concentrated annealing current suificient to heat the metal wall of the tube to its annealing temperature in the form of an annular transverse path around the surface portion of the tube, the Width of the effective heating current path longitudinally of the tube being substantially less than the corresponding Width of the current-inducing coil.

2. In the induction annealing of a metal tube in which the tube is moved longitudinally through a surrounding induction coil in which a high frequency current flows to induce a corresponding heating current in the tube, the improvement which comprises inducing in the tube a concentrated annealing current sufficient to heat the metal wall to its annealing temperature in the form of an annular transverse path around the surface portion of the tube,

the width of the elfective heating current path longitudinally of the tube being substantially less than the corresponding width of the current-inducing coil, and quenching the surface-heated tube sufficiently rapidly to limit the annealing of the tube to substantially only the surface portion thereof.

3. In an induction heating device for annealing a metal tube comprising guide means for guiding longitudinal movement of the tube, an induction coil surrounding the tube, and a source of high frequency current connected to the coil, the improvement which comprises an induction coil consisting essentially of a single turn of a hollow internally cooled conductor having a triangular cross-section positioned with the apex of the triangle facing the tube, and a high-permeability magnetic core surrounding all sides of the hollow conductor except the apex thereof.

4. In an induction heating device for annealing a metal tube comprising guide means for guiding longitudinal movement of the tube, an induction coil surrounding the tube, and a source of high frequency current connected to the coil, the improvement which comprises an induction coil consisting essentially of a single turn of a hollow internally cooled conductor having a triangular cross section positioned with the apex of the triangle facing the tube, a high-permeability magnetic core surrounding all sides of the hollow conductor except the apex thereof, and a thin heat-resisting electrically non-conductive sleeve interposed between the surface of the tube and the apex of the conductor and the immediately adjacent portion of the magnetic core surrounding the conductor.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,229 Roberds Sept. 24, 1946 2,542,392 Chapman Feb. 20, 1951 2,648,759 Machian et a1 Aug. 11, 1953 2,785,263 Van Iperen Mar. 12, 1957 UNITED STATES PATENT cFFTeE CERTIFIGATE 0F CORRECTION Patent No. 3,060,067 October 23 1962 James M, Kennedy, Jr,

peers in the above numbered pat uld read as rtified that error ap (1 Letters Patent sho It is hereby ce tion and that the seLi ent requiring correc corrected below.

Column 6, line 17, for the patent number "2,542,392." read Signed and sealed this 29th day of January 1963.

(SEAL) Attestz' ERNEST w. SWIDER I DAVID LADD Commissioner of Patents 7 Attesting Officer UNITED TATES FATENT GFFEEE CERTIFKQATE OF COP; Q'HN October 23 1962 Patent No. 3,060,067

James M. Kennedy, Jr.

rtified that error appears in the above numbered pat Patent should read as It is hereby ce tion and that the said Letters ent requiring correc corrected below.

Column 6, line 17, for the patent number "254%392. read 1 1393. I

Signed and sealed this 29th day of January 1963,

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of Patents 

1. IN THE INDUCTION ANNEALING OF A METAL TUBE IN WHICH THE TUBE IS MOVED LONGITUDINALLY THROUGH A SURROUNDING INDUCTION COIL IN WHICH A HIGH FREQUENCY CURRENT FLOWS TO INDUCE A CORRESPONDING HEATING CURRENT IN THE TUBE, THE IMPROVEMENT WHICH COMPRISES INCLUDING IN THE TUBE A CONCENTRATED ANNEALING CURRENT SUFFICIENT TO HEAT THE METAL WALL OF THE TUBE TO ITS ANNEALING TEMPERATURE IN THE FORM FIG -01 OF AN ANNULAR TRANVERSE PATH AROUND THE SURFACE PORTION OF THE TUBE, THE WIDTH OF THE EFFECTIVE HEATING CURRENT PATH LONGITUDINALLY OF THE TUBE BEING SUBSTANTIALLY LESS THAN THE CORRESPONDING WIDTH OF THE CURRENT-INDUCING COIL. 